1. Field of the Invention
The present invention relates to multichannel receivers, and, more particularly to multichannel receivers having an analog signal preprocessor which is capable of eliminating signal sidelobes from a received signal frame and a method for doing the same.
2. Description of the
Presently, the electromagnetic signal environment includes an increasing waveform complexity as well as increasing emitter density in both the frequency and time domains. As the emitter density and waveform complexity proliferate, the capabilities of conventional receivers are overwhelmed. It is not unusual for a signal environment to consist of tens of millions of pulses of radio-frequency (RF) data per second. This amount of data requiring processing exceeds the capabilities of present receivers and digital signal processors by a significant number of samples of data per second. These high data rates occur because in order to obtain high probability of intercept of some high priority signals which may have short on-times, present receivers must have high sensitivity and either constant monitoring of the frequencies of interest using many parallel receiver channels or a very wide instantaneous bandwidth scanning receiver which permits rapid revisit to the short on-time frequencies. In either case, the receiver is forced to collect very large amounts of data due to the large instantaneous RF bandwidth used. These complex receivers are generally large, costly, difficult to maintain and consume large amounts of power.
New receiver architectures have been developed specifically to address the dense emitter environment. These architectures include a channelized receiver, a microscan receiver and an acousto-optic (Bragg cell) receiver. Each of these receivers has the disadvantage of producing signal sidelobes associated with valid received signals in both the frequency and time domains. Generally, in these systems, the baseband response to an input signal consists of a train of pulses corresponding to antenna pattern mainlobe responses for a succession of scans. The baseband response during the interpulse periods may contain minor pulse responses, termed time sidelobe responses or ringing effect, which may be mistaken for mainlobe responses by the baseband processor. Additionally, channelized receivers can also produce sidelobe responses as a function of frequency. A channel several frequency channels away from the channel which receives the signal most strongly can also respond to the signal due to signal level of the edges of the channel passband.
Multichannel receivers frequently use analog filtering techniques to perform an initial sorting by frequency, but subsequent processing stages are often overwhelmed by the data flowing through the analog filter. In some instances, up to 200 frequency channels having better than a 10 nanosecond time resolution will be required. In spite of recent advances in digital processing, it remains impractical to digitize and process these large amounts of data due to the extensive amount of hardware needed. Accordingly, in order to digest and utilize the immense quantity of data being received, especially within the size, weight and power constraints of on-board systems, advanced analog preprocessors need to be developed. An advanced analog preprocessor preferably would make rapid decisions on the raw data and transmit only specifically selected information to the digital processing stages.
One form of a modern receiver architecture has been developed by AIL Systems, Inc. of Eaton Corporation, and is described in U.S. Pat. No. 4,652,879, by Rudish, et al , entitled, "Phased Array Antenna System To Produce Wide-Open Coverage Of A Wide Angular Sector With High Directive Gain And Strong Capability To Resolve Multiple Signals" issued Mar. 24, 1987 the disclosure of which is incorporated herein by reference. The Rudish, et al. patent discloses a phased array antenna system which is capable of scanning at very fast rates to prevent the loss of information during the scanning process. The phased array is configured to add the capability to provide multi-dimensional separation of multiple signals to enable measurement of direction of arrival and frequency, and to eliminate the sensitivity loss due to sampling usually encountered with rapid-scan systems.
The system utilizes a technique which processes segments of RF signals to simultaneously achieve frequency channelization and provide angle-of-arrival information as relative timing within a signal segment. This simultaneous frequency and time domain processing introduces ambiguity in the form of time and frequency domain sidelobes that can mask other valid signals within the signal segment. Generally, this system is a combined compressive and channelized receiver and thus produces signal sidelobes in both the frequency and time domains which are difficult to eliminate using conventional analog or digital devices. Accordingly, it would be advantageous to provide such a system with an advanced analog preprocessor having the capability of eliminating signal sidelobes and thus enabling the system to process all valid signals received.